Solid polymer electrolyte and electrochemical cell including said electrolyte

ABSTRACT

A solid polymer electrolyte is provided including a solid solution of a  lium salt such as LiCF 3  SO 3  in a polymer host such as poly(ethylene oxide). The electrolyte also includes a small amount of a plasticizing agent such as bis(2 ethylhexyl) sebacate. The electrolyte can be used to make a solid state electrochemical cell.

GOVERNMENT INTEREST

The invention described herein may be manufactured, used, and licensedby or for the Government for governmental purposes without the paymentto me of any royalties thereon.

FIELD OF INVENTION

The invention relates to solid, ionically conductive polymers and totheir use as electrolytes in electrochemical cells.

BACKGROUND OF THE INVENTION

Solid polymer electrolyte (SPEs) containing dissolved metal salts havebeen proposed as an alternative to liquid electrolytes in rechargeableelectrochemical devices. There are many advantages to using a solidelectrolyte, such as the capability for high speed production of thincells constructed in a bipolar arrangement. Further, the electrolyte canact as a mechanical separator between the anode and cathode, eliminatingthe need for an inert porous separator, as well as acting as abinder/adhesive to move and conform to electrode volume changes duringcycling. Because the system would be all solid state, cells of anydesired geometric shape would be possible. There is also a safetyadvantage in that if the integrity of the sealed cell package is broken,there is no liquid to leak out.

One polymer system of interest is based on high molecular weightpoly(ethylene oxide), HO--CH₂ CH₂ (O--CH₂ CH₂)_(n) OH. An ionicallysolid polymer electrolyte can be prepared by dissolving PEO and anappropriate salt in a suitable volatile solvent such as acetonitrile. Bysolution casting, acetonitrile is removed by evaporation, leaving a freestanding, solid, flexible film of good mechanical strength that containsonly PEO with dissolved salt. Such films are ionic conductors.Unfortunately, at room temperature, these PEO films are highlycrystalline so ionic conductivity is poor (≈10⁻⁷ Scm⁻²) rendering PEO asan impractical electrolyte at these temperatures.

SUMMARY OF THE INVENTION

The general object of this invention is to provide a solid, ionicallyconductive polymer that can be used as an electrolyte in electrochemicalcells. A more particular object of the invention is to improve the ionicconduction of a typical host polymer such as PEO with a dissolvedlithium salt such as LiClO₄, LiBF₄, LiCF₃ SO₃, or LiAsF₆ so that it canbe used as an electrolyte in a solid state electrochemical cell.

It has now been found that the aforementioned objects can be attained bythe addition of a plasticizing agent to increase the amorphous characterof the polymer host and thereby enable increased ionic conductivity. Onesuch material found to act as an effective plasticizer for PEO is bis (2ethylhexyl) sebacate, or "dioctyl" sebacate. The addition of dioctylsebacate to (PEO)₂₀ (LiCF₃ SO₃), causes an immediate increase in ionicconductivity between room temperature, RT and the melting temperature of(PEO), T_(m). The increase is equal to or greater than that observed bytemperature-cycled polymer without dioctyl sebacate. In polymercontaining dioctyl sebacate, conductivities for first (temperature)cycle and subsequent cycles are indistinguishable.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows the Log Conductivity vs. Temperature for the FirstTemperature Cycle for films of [PEO]₂₀ [LiCF₃ SO₃ ]±[Dioctyl Sebacate].

FIG. 2 shows the Log Conductivity vs. Temperature Subsequent to theFirst Temperature Cycle for films of [PEO]₂₀ [LiCF₃ SO₃ ]±[DioctylSebacate].

DESCRIPTION OF THE DRAWING AND THE PREFERRED EMBODIMENT

Poly (ethyleneoxide), HO--CH₂ CH₂ (O--CH₂ CH₂)_(n) OH (average molecularweight of 4×10⁶, dried at 50° C. under vacuum overnight) and lithiumtrifluoromethanesulfonate (LiCF₃ SO₃, dried at 100° C. under vacuum) inmolar ratios of 20:1, respectively, are dissolved in acetonitrile(distilled under a stream of dry argon) with stirring in an argon-filledglove box containing less than 10 ppm water vapor. A second solution isprepared with bis(2 ethylhexyl) sebacate to yield a molar ratio of20:1:1. Films are cast by pouring the solutions into flat teflon dishes.After allowing the solvent to completely evaporate, free-standing films(50 to 100 μm thick) are peeled from the dishes. Films are placedbetween stainless steel blocking electrodes and conductivities arecalculated using ac impedance measurements taken from 5 Hz to 100 kHzwith an EG&G PAR Model 388 Electrochemical Impedance System.

The ionic conductivity of films containing bis(2 ethylhexyl) sebacate issignificantly higher at temperatures below T_(m) compared to filmswithout the additive as shown in FIG. 1 (the plasticized PEO data is formore than one film). It appears that conductivity is enhanced slightlyat temperatures above T_(m) as well. This renders the film morepractical for use in an electrochemical cell since batteries areordinarily used at room temperature. Further improvements by theaddition of other salts or plasticizers and refinements in the molarratios used may produce films with even higher conductivities.

FIG. 2 shows conductivity vs temperature for temperature cyclessubsequent to the first cycle. Plasticized films maintain comparable orbetter conductivity than un-plasticized films. In films containingdioctyl sebacate, conductivity for first (temperature) cycle andsubsequent cycles are indistinguishable.

Since Li⁺ associates with oxygen molecules on PEO, it is possible thatbis(2 ethylhexyl) sebacate performs a dual role, increasing theamorphous nature of PEO and also complexing/coordinating lithium ionswith the oxygen molecules on itself. Other substances of similarconfigurations might also be expected to be useful in augmenting ionicconductivity.

In lieu of poly(ethylene oxide) as the polymer host, one may use atleast one of the following, poly(propylene oxide) and mixtures ofpoly(ethylene oxide) and poly(propylene oxide).

Similarly, other plasticizing agents may be used in the invention suchas LiN(CF₃ SO₂)₂, propylene carbonate-ethylene carbonate mixtures,triethylene glycol dimetharylate, and esters of phthalic, adipic, andphosphoric acids.

Though the preferred embodiment shows a molar ratio ofPEO:salt:plasticizer of 20:1:1, alternate ratios may be used to optimizeperformance characteristics.

The anode of an electrochemical cell including the solid polymerelectrolyte and plasticizer may be either lithium, a lithium alloy, or alithium intercalate such as LiC₆, graphite, or petroleum coke. Thecathode may be a metal oxide such as Ag₂ CrO₄, CuO, Bi₂ O₃, Bi₂ Pb₂ O₅,CrO_(x), MnO₂, Li_(x) MnO_(y), MoO₃, MoO₃, LiNiO₂, V₂ O₅, V₆ O₁₃, LiCoO₂or sulfide such as CuS, FeS_(x), TiS₂, MoS₂, Cr_(x) V_(1-x) S₂, Ni₃ S₂,or fluoride such as CuF₂ (CF)_(n) or chloride such as CuCl₂, AgCl oracetylene black carbon, or lithium intercalating compound, orelectrically conductive polymer such as polyactylene, poly(alkyl-thiophene), polyaniline, phenylene, phenylene sulfide, ormixtures thereof; including a solid polymer electrolyte. Theelectrochemical cell containing such an anode, solid polymer electrolytecontaining plasticizer, and cathode may be primary (nonrechargeable) orsecondary (rechargeable).

I wish it to be understood that I do not desire to be limited to theexact details of construction shown and described for obviousmodification will occur to a person skilled in the art.

What is claimed is:
 1. A solid polymer electrolyte including a solutionof at least one lithium salt in at least one polymer host, and whereinsaid electrolyte includes a plasticizing agent.
 2. A solid polymerelectrolyte according to claim 1 wherein said polymer host is at leastone polymer selected from the group consisting of poly(ethylene oxide)(PEO), polyacetylene, poly(alkyl-thiophene), polyaniline, phenylene, andphenylene sulfide, wherein said lithium salt is at least one saltselected from the group consisting of LiClO₄, LiBF₄, LiCF₃ SO₃, and LiAsF₆ and wherein said plasticizing agent is selected from the groupconsisting of bis(2 ethylhexyl) sebacate, mixtures of propylenecarbonate and ethylene carbonate, triethylene glycol dimethacrylate,esters of phthalic acid, esters of adipic acid, and esters of phosphoricacid.
 3. A solid polymer electrolyte according to claim 2 wherein saidpolymer host is poly(ethylene oxide), said lithium salt is LiCF₃ SO₃,and said plasticizing agent is bis(2 ethylhexyl ) sebacate.
 4. A solidpolymer electrolyte according to claim 3 wherein the ratio by weight ofpoly(ethylene oxide):LiCF₃ SO₃ : bis(2 ethylhexyl sebacate) is 20:1:1.5. An electrochemical cell including an anode selected from the groupconsisting of lithium metal, lithium alloy, LiC₆, graphite, andpetroleum coke, a cathode selected from the group consisting of Ag₂CrO₄, CuO, Bi₂ O₃, Bi₂ Pb₂ O₅, CrO_(x), MnO₂, Li_(x) MnO_(y), MoO₃,LiNiO₂, V₂ O₅, V₆ O₁₃, LiCoO₂, CuS, FeS₂, TiS₂, MoS₂, Cr_(x) V_(1-x),S₂, Ni₃ S₂, CuF₂, (CF)_(n), CuCl₂, and AgCl and a solid polymerelectrolyte including a solid solution of at least one lithium salt inat least one polymer host, and said electrolyte including a plasticizingagent.
 6. An electrochemical cell according to claim 5 wherein the solidpolymer electrolyte includes a solid solution of LiCF₃ CO₃ inpoly(ethylene oxide); the solid polymer electrolyte also including bis(2ethylhexyl) sebacate.
 7. A rechargeable electrochemical cell accordingto claim 6 wherein the polymer is poly(ethylene oxides), wherein thesalt is LiCF₃ SO₃, and wherein the plasticizing agent is bis(2ethylhexyl) sebacate.
 8. An electrochemical cell according to claim 6wherein the cell is a rechargeable cell.
 9. An electrochemical cellaccording to claim 7 wherein the cell is a primary cell.